Drilling rig depth control

ABSTRACT

A system which monitors the depth at which a drilling rig is operating and indicates when a predetermined depth is reached by recording the length traveled by the drill pipe supporting member in a predetermined direction within the mast of the drilling rig, comparing the length traveled with a predetermined length, distance or measurement and producing a signal when the two are equal. This may be a prealarm signal or a final alarm signal. Preferably the signal sounds an alarm and automatically interrupts power to the drill pipe supporting member.

United States Patent Greene [54] DRILLING RIG DEPTH CONTROL [72] Inventor: Palmer G. Greene, Dallas, Tex.

[73] Assignee: Automatic Drilling Machines, Inc., Dallas,

Tex.

[22] Filed: Apr. 17,1970

[2i] Appl.No.: 29,469

[52] US. Cl ..l73/2l, 33/132,175/40 [51] Int. Cl ..E2lb 47/04 [58] Field 01 Search ..33/132134;

[56] References Cited UNITED STATES PATENTS 3,522,727 8/1970 Calhoun ..73/l51.5

[451 Mar. 28, 1972 3,052,878 9/1962 Berry ..l75/40 X Primary Examiner-Ernest R. Purser Attorney-Richards, Harris & Hubbard [5 7] ABSTRACT A system which monitors the depth at which a drilling rig is operating and indicates when a predetermined depth is reached by recording the length traveled by the drill pipe supporting member in a predetermined direction within the mast of the drilling rig, comparing the length traveled with a predetermined length, distance or measurement and producing a signal when the two are equal. This may be a prealarm signal or a final alarm signal. Preferably the signal sounds an alarm and automatically interrupts power to the drill pipe supporting member.

6 Claims, 3 Drawing Figures DEPTH INDICATOR AND CONTROLLER PATENTEDMAR28 m2 3,651,871

sum 1 [1F 2 50 34 38 W 52 54 V 3O ZAMI? a: RATE AMP. MUD PUMP 88 90 /04 I00 1 /6/92 94 \J 5 7 L Ill I25 l J I \I I /24 I23 DEPTH INDICATOR I44 I26 AND CONTROLLER /06 A //2 INVENTOR:

PALMER G. GREENE ifi T //4 1 fluwdfiw -wd ATTORNEYS DRILLING RIG DEPTH CONTROL BACKGROUNDOF THE INVENTION This invention relates to drilling rig control, and more particularly to a determination of the depth at which the drilling rig is operating.

In drilling through earth formations various conditions can occur which may damage the equipment employed in the drilling operation. For example, during the drilling operation there may develop bridges," ledges" or keyseats" which, upon withdrawing or inserting the drill bit from or into the borehole may cause damage to the drilling equipment. A keyseat," which is a slot formed in the borehole by a drill string which is exerting a lateral pressure on the formation can cause the drill string to part as it is retrieved from the hole. More particularly, as the drill bit is commonly wider than the drill string, it can wedge in the slot or keyseat" upon being retrieved from the hole creating a tension on the drill string which, if undetected, can cause the drill string to sever, collapse the mast of the drilling rig or damage the drill string lifting mechanism.

The present invention provides means for indicating when a drilling rig has reached a predetermined depth at which danger areas are known to exist and for generating an alarm signal and for shutting down the drill string lifting mechanism when the predetermined depth is reached.

The invention may be generally described as a system for monitoring the depth at which a drilling rig is operating. The system includes first means which are operably associated with the drill pipe supporting member of the drill rig to record the length traveled by the member in a predetermined direction. Second means are provided for comparing the length traveled by the drill string supporting member with a predetermined distance. Third means produce a signal when the length traveled by the drill pipe supporting member is equal to the predetermined distance. Means are also preferably provided for interrupting power for stopping movement of the drill pipe supporting member when the third means produce a signal indicating that a predetermined depth has been reached.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic of a drilling rig and four subsystems for detecting off-normal drilling operation;

FIG. 2 is an electrical schematic of a system for indicating to a rig operator off-normal operation and for shutting down a drilling rig when a predetermined depth has been reached; and

FIG. 3 is a more detailed electrical schematic of a portion of the depth control means of FIG. 1.

DESCRIPTION OF A PREFERRED EMBODIMENT As the present invention is preferably utilized in conjunction with means for monitoring and indicating when a drill I string has encountered an off-normal condition, it is illustrated and described in connection with such equipment. Specifically, once other means have, on a trip in or out of the borehole, detected areas where conditions occur that could be injurious to the equipment, the present invention may be utilized to indicate the depth at which this condition occurs and be also utilized to warn the operator on the next trip in or out that he is about to reach one of these trouble areas.

More specifically, and with reference to FIG. 1, there is shown an application of the present invention to an automatic hydraulic drilling rig of the type described in U. S. Pat. No. 3,158,213. The drilling rig includes a mast 12 within which is slidably carried a vertically movable drill head assembly 14. Drill head assembly 14 supports a drill string 16, only a portion of which has been illustrated, that may be several thousand feet in length, by means of elevators (not shown) or a threaded spindle 76 which extends from assembly 14. Movement of the drill head assembly 14 relative to the mast 12 is accomplished by a suitably braced tower 18 fixed to the top of the mast 12. The tower 18 includes cylinders 20 and 22, more than two may be used, which are braced by struts 24. Rods 26 and 28, which are mounted to pistons within the cylinders 20 and 22, respectively, efi'ect vertical movement of the drill head upon introduction of hydraulic fluid into the cylinders through pipes 30 and 32.

Fluid flow rates within the pipes 30 and 32 may be controlled by a hydraulic system of the type described in the copending U.S. Pat. application of Raymond J. Bromell, et al., Ser. No. 782,231, filed Dec. 9, I968, and assigned to the assignee of the present invention. As described in this copending application, the yoke of a variable displacement pump is stroked to various positions to establish flow rates in the pipes 30 and 32 and thus vary the speed of raising and lowering the drill head assembly 14. The pressure maintained in the cylinders 20 and 22 by the variable displacement pump will be at a level established by the weight of the drill string 16.

Pressure transducers 34 and 36 are coupled to the pipes 30 and 32, respectively, and produce electrical signals proportional to the pipe pressure. Since there are two or more cylinders lifting the drill head assembly 14 and drill string 16, a pressure in the pipes 30 and 32 will be proportional to only a portion of the total weight of the drill head and drill string. Thus, the electrical signals from the transducers 34 and 36 are added in a summing amplifier 38 to produce a signal on a line 40 proportional to the weight of the drill string 16. The electrical signal on the line 40 is coupled to an indicating meter 42.

In addition to generating electrical signals representing drill string weight by converting pressure in the cylinders 20 and 22, other transducers may be employed. For example, strain gauges may be mounted on the rods 26 and 28 to produce electrical signals representing the drill string weight. Further load cells may be mounted under the rig to produce electrical signals that vary with drill string weight.

The signal on line 40 is tied to an alarm controller 50. A typical controller 50 may include a potentiometer circuit coupled to a knob 52 for establishing a signal representing the calculated weight of the drill string 16. The alarm controller may also include an amplifier having a first output when the drill string weight is above a calculated value, a second output when the actual weight is at the calculated value, and a third output when the actual'weight is below the calculated value. The first and third outputs of the amplifier energize a relay 54 to complete various alarm circuits and shut down the drilling rig operation, as will be described shortly. When the first output energizes the relay 54, a high alarm condition exists such as would occur when a reduced hole, sloughing shale or keyseat is encountered when removing the drill string from a hole. Energizing the relay 54 from the third output indicates a low" alarm condition such as would occur for a drill string twist-off or when encountering a ledge or bridge when lowering the drill string into a hole.

A signal on line 40 is also coupled to a pressure v. time rate amplifier 56 which energizes the relay 54 when the rate of change of the drill string weight exceeds an established level. Many rate amplifier circuits are available and additional description is not deemed necessary.

To indicate off-normal drilling rig operation with the weight alarm circuit described above, the knob 52 is set at a drill string weight calculated to be slightly less than the weight being supported. Pressure in the pipes 30 and 32 will be maintained at a level to support the drill string and will remain at some reasonably constant value for a given weight. Assume the drill string 16 is about 2,000 feet long and a twist-off occurs at about the 1,000 foot level, then the pressure in pipes head assembly from running to the top of and damaging the tower. A lower pressure in the pipes 30 and 32 produces a signal of lower magnitude on the line 40 that will be less than a signal established by the knob 52. As a result, the alarm controller 5O energizes the relay 54, thereby closing a contact 54-1 in series with an indicating light 60 shown in FIG. 2. The light 60 will be connected to a source of voltage at the terminals 62 and 64 through a contact 66. The contact 66 has previously been closed in the normal sequence of operation. In addition to turning on the light 60 by closing the contact 54-1, energizing the relay 54 closes a contact 54-2 to energize a horn 68 to produce an audible alarm.

In addition to giving a visual and audible indication of a change in drill string weight, energizing the relay 54 also opens a contact 54-3 which de-energizes a relay 70, thereby shutting down the drilling rig. The relay 70 is energized at the start of the drilling operation by closing the switch 72 and pushing the reset button 74. This completes a circuit to the terminals 62 and 64 through a normally closed contact 66-2. Contact 66-2 is opened after the sequence of activating the drilling rig has been completed. The relay 70 will be energized through other normally closed contacts, to be described. Energizing the relay 70 closes contacts 70-1 and 70-2. Contact 70-1 completes a holding circuit for the relay 70 and the contact 70-2 connects drilling rig control circuitry 72 to the terminals 62 and 64.

For the alarm controller 50 to properly control the relay 54, the knob 52 must be adjusted for each additional length of drill pipe added to the drill string 16. This can be accomplished by hand or automatically by a computer. However, should adjustment of the knob 52 be overlooked, the weight alarm circuitry will give an indication of off-normal rig operation by means of the rate amplifier 56. The rate amplifier 56 energizes the relay 54 when the rate of change of drill string weight exceeds an established value. Under normal operating conditions for the drilling rig, that is, during the drilling operation or when lowering the drill string into a hole, the rate of change will be below the established level. When a drill string twist-off or other malfunction occurs, however, the rate of change of drill string weight will be high, thereby energizing the relay 54. Energizing the relay 54 closes contacts 54-1 and 54-2 to activate the light 60 and the horn 68, respectively. In addition, energizing the relay 54 opens a contact 54-3, thereby de-energizing the relay 70. This, as explained, shuts down the drilling rig operation.

In addition to indicating drill string twist-off, the weight alarm circuitry will indicate a ledge" condition before any damaging amount of weight has been applied to a drill bit when lowering a string into a previously formed hole. The knob 52 is set, by hand or automatically, at a drill string weight slightly lower than the pickup weight, that is, the hanging weight of the string. 1f the drilling bit comes in contact with a ledge, the pressure in the pipes 30 and 32 will suddenly decrease, thereby setting up the same conditions that were described previously with regard to a twist-off. The light 60 and the horn 68 will be activated and the relay 70 de-energized to shut down the drilling rig, thus preventing bending of the drill string and damage to the drill bit. This additional feature of the weight control circuitry shuts down the rig before an excessive amount of weight damages the drill string or the drill bit.

Still another feature of the weight alarm circuitry, the drilling rig will be shut down before the drill bit binds in a tight" hole or keyseat when removing the drill string from a hole. For this feature, the knob 52 is set at a drill string weight slightly higher than the pickup weight of the string. The setting of the knob 52 should not, however, be at a level such that the relay 54 will be energized. A tight hole or keyseat" condition will be detected when removing the drill string by the high alarm (the third output) signal from the controller 50 which energizes the relay 54. As the drill bit encounters a keyseat or tight" hole, the pressure in the pipes 30 and 32 will rapidly increase, thereby generating the high alarm signal from the controller 50. Energizing the relay 54 will activate the light 60 and the horn 68 by closing contacts 54-1 and 54-2, respectively. The relay 70 will be de-energized by opening the contact 54-3 to shut down the rig, thus minimizing the possibility of pulling the drill string in two or collapsing the mast.

Another variable that provides an indication of off-normal drilling rig operation is the torque required to rotate the drill string. The drill head assembly 14 includes one or more fluid motors 73 coupled through pinion and gear mechanisms to a pipe gripper section 76. A complete description of the drill head assembly 14 will be found in the U.S. Pat. No. 3,158,213. Hydraulic fluid for operating the motors 73 will be provided through flexible hoses, such as the input hose 78 and the return hose 79. The torque alarm circuit includes a transducer 80 for converting the pressure in the hose 78 to an electrical signal on a line 82. The signal on line 82 therefore represents the pressure in the hose 78 and in turn the torque produced by the hydraulic motor 73. The signal on line 82 is coupled to an indicating meter 84 having a scale 86 calibrated in units of torque. A recorder 88 also connects to the line 82 and produces a permanent record of the torque required for rotating the drill string 16.

To indicate an off-nonnal operation, e.g., a probability of drill string twist-off," the signal on line 82 connects to an alarm controller 90 which may include an amplifier having a first output when the signal on line 82 is above a minimum setting and a second output when this signal drops below the setting. The alarm controller 90 includes a knob 92, which may be set by a computer, for setting a minimum and maximum value of torque to be developed by the hydraulic motor 73. Any change in torque produced by the motors 73 above or below the setting on controller 90 will energize a relay 94.

Energizing the relay 94 closes a contact 94-1 to connect a light 96 to the terminals 62 and 64. Energizing the relay 94 also closes a contact 94-2 and opens a contact 94-3 in the circuit for the horn 68 and relay 70, respectively. Note, closing the contact 94-2 does not activate the horn 68 and opening the contact 94-3 does not de-energized the relay 70. Since a decrease in torque is not a positive indication of a drill string twist-oft with the system illustrated, only the light 96 is turned on to indicate to the operator a probable twist-off or that some other malfunction has occurred.

in addition to indicating a drill string twist-off, a change in torque may also indicate a dull drill bit. As the drill bit wears with usage, additional torque will be required to penetrate the subsurface layers. When this increase in torque exceeds the value set by the knob 92, the relay 94 will be energized, thereby indicating an off-normal operation.

For a sudden decrease in torque to activate the horn 68 and de-energize the relay 70, there must be an accompanying nonprogrammed decrease in mud pump pressure. Drilling mud may be pumped into the drill string 16 via a mud tube 98 and a hollow portion of the section 76. A hose 100 connects to the mud tube 98 and a mud pump (not shown). The mud pump pressure alarm circuit includes a transducer 102 connected in the hose 100 to convert the mud pump pressure to an electrical signal on line 104. Usually, the transducer 102 will be installed in an oil chamber which is separated from the drilling mud by a flexible diaphragm. Such transducers are readily available and additional description is not deemed necessary.

The electrical signal representing mud pump pressure on the line 104 connects to an indicator/alarm meter 106 or to a controller, such as controller 90. An indicator/alarm meter suitable for the system shown includes an indicating needle 108 and a set point needle 110. When these two needles coincide, they operate electrical contacts to energize a relay 112 from an electrical power source 114. Energizing the relay 112 closes a contact 112-l, as shown in FIG. 2, to connect a light 116 to the terminals 62 and 64. Energizing the relay 112 also closes a contact 112-2 and opens a contact 112-3 for the horn 68 and the relay 70, respectively. Closing the contact 112-2 will not actuate the horn 68 and opening the contact 112-3 will not de-energize the relay 70 unless the relays 94 and 112 are energized at theysame time. If the relays 94 and 112 are energized together, then the contacts 94-2 and 112-2 will be closed and the horn 68 will be energized to give an audible signal to an operator that a drill string twist-off" has occurred. Opening the contacts 94-3 and 112-3 at the same time will disconnect the relay 70 from the terminals 62 and 64, thereby shutting down the drilling rig by opening the contact 70-2.

By monitoring mud pump pressure, off-normal operations other than a drill string twist-off bay be detected. For example, if the drill string develops holes, the drilling mud will be lost through these holes and the mud pump pressure will drop. By substituting the controller 90 for the meter 106, a high mud pump pressure will cause an energization of the relay 112. High mud pump pressure will result when the drill bit becomes plugged or when a sloughing shale condition develops. When a drill bit becomes plugged, it is important that the drilling be stopped to avoid damaging the bit. Not all deviations of mud pump pressure from a normal setting, however, require a complete shutdown of the rig operation. In fact, it may be desirable to continue the flow of drilling mud at some nominal value. In a sloughing shale condition, the mud pump pressure will be reduced to avoid damage to the hole through fracturing by excessive pressure, but some drilling mud flow is desirable to prevent the shale from packing around the drill string.

Although the circuit of FIG. 2 illustrates that a drill string weight change is an indication of a drill string twist-off independent of the other variables, it should be understood that by modifying the contact arrangement, a twist-off" indication may not be given unless there was a change in drill string weight accompanied by deviation of one of the other variables from their established levels. By various contact arrangements, an indication of drill string twist-off can be made to be dependent upon a deviation from established levels of any combination of two of the three variables described. Thus, it should be understood that the contact arrangement described illustrates only one embodiment and is not intended as a limitation to the system shown.

The drilling rig which is partially illustrated in FIG. 1 includes means for indicating the depth at which the drilling rig is operating when the drill bit encounters a ledge, bridge or keyseat" as well as at what level the rig is operating when a twist-off occurs.

More particularly, these means take the form of a footage switch 116 mounted on the drill head assembly 14. Upon vertical movement of drill head assembly 14, switch 116 is actuated by a plurality of cams 1 17 spaced at l-foot intervals along leg 118 of mast 12. Drill head assembly 14 also has attached thereto a direction switch 119 which is adapted to be actuated by cams 121 and 122 which are slightly larger than cams 1 17.

Both of the switches 116 and 119 communicate through conductor bundle 123 with a depth indicator and controller 124 which is coupled to a recorder 125 for preserving in permanent form a record of the depth of the rig as a function of time and is also coupled to a relay 126, the purpose of which will be explained hereinafter. Depth controller 124 and the switches which are responsible for admitting signals thereto are illustrated in more detail in FIG. 3. In FIG. 3, the footage switch 116, which is for example a single-pole double-throw toggle switch, has an arm 127 for engagement with cams 117. As drill head assembly 14 moves down the mast 12 arm 16 will engage cams 117 and move upward to engage contact 128. Conversely, when drill head assembly 14 moves up the mast l2, arm 127 will, upon encountering one of the cams 117, move downward to engage contact 129. Arm 127 is spring biased to the center position, as illustrated in FIG. 3 so that it returns to this position after engaging each of the cams 117 regardless of the direction of travel of the drill head assembly 14.

Switch 119 is a single-pole double-throw non-centered switch having an arm 13] which, upon engagement with cam 121, at which point drill head assembly 14 will be at its uppermost elevation, will be deflected to engage contact 132, as illustrated in FIG. 3. Conversely, when drill head assembly 14 reaches its lowermost position, arm 131 will engage cam 122 and be moved upwardly to engage contact 133. Switches 116 and 119 serve, in cooperation with a double-pole single-throw switch 134 to complete a circuit to a pulse counter 136, such as a Veeder-Root counter, Series 1847 manufactured by the Veeder-Root Company, Hartford, Connecticut. The pulse counter is provided with first and second registers 137 and 138. Second register 138 is provided with a set of manually adjustable knobs 139 to permit entry within the register 138 of a desired number. The output of register 137 is coupled by conductor 141 to the recorder and a comparator 142. Comparator 142 also receives an input through conductor 143 from register 138 to compare the relative magnitude of the numbers in registers 137 and 138 and to generate an output signal in conductor 144 when the values of the two registers 137 and 138 are equal for energizing the relay 126.

The register 137 communicates through conductor 145 with contacts 128 and 129 of switch 116. Thus, a signal in conductor 145 will be transmitted, upon contact of center arm 127 with either of contacts 128 or 129 due to engagement of arm 127 with one of the cams 117, to conductor 146 which communicates with center arm 131 of switch 119. A signal transmitted through contact 132 or contact 133 will pass through conductor 147 or conductor 148, depending upon the position of arm 131, to anns 149 and 151 of switch 134.

When the drilling rig is making a trip into the hole with drill pipe, switch 134 is manually set to the position indicated in FIG. 3, in which event arm 149 will communicate with register 137 through conductor 152 whereas arm 151 only communicates with an open contact. With switch 134 in the second position, as for example when the device is making a trip out of the borehole, the signal in conductor 148 and am 151 will communicate through conductor 153 with register 137 whereas the signal and arm 145 will be to an open contact.

In operation, if the operator of the drilling rig wishes to count the number of feet of drill pipe in the hole on a trip into the hole, switch 134 is manually moved to the position indicated in FIG. 3 and drill head 14 is moved (by means not shown) to the top of mast 12. As drill head assembly 14 reaches the top of mast l2 cam 121 will engage arm 131 moving it into engagement with contact 132, as illustrated in FIG. 3. As drill head assembly 14 is lowered to admit pipe to a borehole, arm 127 will, upon engaging the cams 117, momentarily engage contact 128 completing the circuit through conductor 145, switches 116 and 119 to conductor 147, arm 149 and conductor 152. The closed circuit will cause register 137 to enter a unit in the register for each pulse caused by closing of arm 127 against contact 128. Register 137 will, as cams 117 are spaced at l-foot intervals, register the number of feet admitted to the borehold. When drill head assembly 14 reaches the bottom of the mast 12, center arm 131 of switch 119 will engage cam 122 moving the arm 131 into engagement with contact 133. As drill head assembly 14 is then elevated upward in mast 12 to receive another joint of drill pipe the pulse which is generated upon the contact of arm 127 with contact 129 will not be counted as the signal will communicate through conductor 148 with the arm 151 which engages an open contact. Upon reaching the uppermost position drill head assembly 14 will again move arm 131 into engagement with contact 132 so that the pulses are again counted as drill head assembly moves downward. If a ledge or the like has been detected on the trip out of the hole or on a previous trip, for example, the 300-foot level, the operator may, on the trip back into the hole, set register 138 by manually inserting the digits 290 of the first three positions in register 138. The register 138 may be set by turning of manually adjustable knobs 139. When the register 137 indicates 290, a signal is transmitted through conductor 141 which is equal in magnitude to the signal in conductor 143. This equality will be sensed by the comparator 142 causing comparator 142 to transmit a signal through conductor 144 to energized relay 126. Energization of relay 126 will warn the operator of the drilling rig in a manner to be later described. The operator may then assume manual control of the drilling rig and slowly move drill head assembly downwardly while closely observing gauges 42 and 84 to assure that the rig is not damaged by this ledge. Once the danger area is passed, register 138 may be set to a lower desired depth and the drilling rig again placed on automatic operation.

If the number of feet of the pipe remaining in the borehole is to be determined on a trip out", switch 134 is moved to the out position in which event, after clearing of register 137 by manipulation of adjustable knobs 140, the number of feet of pipe withdrawn from the hole may be observed as center arm 131 of switch 119 will have been moved into engagement with contact 133 by movement of drill head assembly 14 to its lowermost position causing pulses created by movement of center arm 127 into engagement with contact 129 to be admitted through arm 151 and conductor 153 to register 137. By setting of register 138 in the same manner as described above on a trip into the borehole the values in registers 137 and 138 may be compared and a signal generated by comparator 142 when the signal from registers 137 and 138 are equal for stopping the rig at known ledge" areas or other danger areas.

With reference to FIG. 2, a signal from comparator 142 will interrupt power to drill head assembly 14 as energization of relay 126 closes contacts 126-1 and 126-3 to ignite lamp 120 and sound alarm 68 in the manner described before. Energization of relay 126 also opens normally closed relay 126-2 to the energized relay 70 thereby open contacts 70-2 terminating power from drilling rig control 72.

With the system described above, it is possible, once a danger area is found to exist at a particular point in a borehole to program the depth indicator and controller 124 to stop the drilling rig at a predetermined depth to permit the operator to manually control the rig past the danger to assure that no damage is done to the equipment or stop prior to reaching existing total depth of the hole to permit slow washing to bottom.

It is also possible to determine at what point a twist-off occurs as the length of pipe removed from the hole after twist-off" may be measured to assist in subsequent fishing operations as the point at which the fish may be located within the hole may be fairly accurately determined by the use of the depth indicator and controller.

While specific embodiment of the invention has been illustrated and described, the invention is not limited to that embodiment, but is defined by the following claims.

What is claimed is:

1. In a drilling rig having a mast, and a drill pipe supporting member mounted for vertical movement within said mast for elevating and lowering drill pipe into a hole beneath the mast, a system for monitoring the depth at which the drilling rig is operating and indicating when a predetermined depth is reached, which system comprises:

first means operably associated with said drill pipe supporting member for recording the length traveled by said member in a predetermined direction;

second means operably associated with said first means for comparing the length traveled by said drill pipe supporting member in a predetermined direction with a predetermined distance; and

third means for producing a signal when the length traveled by said drill pipe supporting member in the predetermined direction is equal to the predetermined distance.

2. The system of claim 1 including fourth means operably associated with said third means for interrupting power to said drill pipe supporting member when the length traveled by the drill pipe supporting member is equal to the predetermined distance.

3. The system of claim 1 wherein said first means comprises:

means for generating a pulse each time the drill pipe supporting member travels a unit of length in a predetermined direction;

a pulse counter;

means connecting said pulse generating means with said pulse countenand means for visually indicating the number of pulses generated and thereby the number of units representing the length traveled by the drill pipe supporting member.

4. The system of claim 1 wherein said third means includes:

means for sounding an alarm when the length traveled by said drill pipe supporting member in a predetermined direction is equal to the predetermined distance.

5. The system of claim 4 including fourth means responsive to the signal from said third means for stopping the drill string supporting member.

6. The system of claim 1 including fourth means responsive to the signal from said third means for stopping the drill string supporting member. 

1. In a drilling rig having a mast, and a drill pipe supporting member mounted for vertical movement within said mast for elevating and lowering drill pipe into a hole beneath the mast, a system for monitoring the depth at which the drilling rig is operating and indicating when a predetermined depth is reached, which system comprises: first means operably associated with said drill pipe supporting member for recording the length traveled by said member in a predetermined direction; second means operably associated with said first means for comparing the length traveled by said drill pipe supporting member in a predetermined direction with a predetermined distance; and third means for producing a signal when the length traveled by said drill pipe supporting member in the predetermined direction is equal to the predetermined distance.
 2. The system of claim 1 including fourth means operably associated with said third means for interrupting power to said drill pipe supporting member when the length traveled by the drill pipe supporting member is equal to the predetermined distance.
 3. The system of claim 1 wherein said first means comprises: means for generating a pulse each time the drill pipe supporting member travels a unit of length in a predetermined direction; a pulse counter; means connecting said pulse generating means with said pulse counter; and means for visually indicating the number of pulses generated and thereby the number of units representing the length traveled by the drill pipe supporting member.
 4. The system of claim 1 wherein said third means includes: means for sounding an alarm when the length traveled by said drill pipe supporting member in a predetermined direction is equal to the predetermined distance.
 5. The system of claim 4 including fourth means responsive to the signal from said third means for stopping the drill string supporting member.
 6. The system of claim 1 including fourth means responsive to the signal from said third means for stopping the drill string supporting member. 